1. Field of the Invention
The present invention relates to an image forming system such as a copying machine, printer and the like, having a heat fixing device.
2. Description of the Related Art
In the past, in image forming systems such as electrophotographic recording systems having heat fixing devices, particularly heat roller fixing devices, an operator could not utilize the image forming system until a surface temperature of a heat roller has reached a predetermined level after a power source has been turned ON. Efforts have been made to reduce the waiting time during which the system could not be used after the power source was turned ON. For example, it has been considered to reduce the heat capacity of a heat roller and/or to increase the heating value of a heater for increasing the temperature increasing speed of the heat roller.
However, if the thickness of a roller core made of aluminum and the like is decreased to reduce the heat capacity of the heat roller, heat conduction in a longitudinal direction will be worsened, and further, the heat roller will be more apt to deform from pressure. Furthermore, if the heater is set to compensate for the reduction of the heat conduction, when small-sized recording sheets are printed continuously, since the surface temperature of the portion of the heat roller which does not contact with the recording sheet the, the service life or durability of a separating claw and bearings which contact the heat roller must be increased, thus making the system expensive. Further, since the heat roller is apt to deform, it is not possible to apply adequate force to the heat roller fixing device, thus worsening the fusion of toner on the recording sheet. If the heating value of the heater is increased, since the maximum power consumption is also increased, for example, when a plurality of image forming systems are connected to a power source via a single plug socket, the number of systems to be connected is limited.
Further, as disclosed in the Japanese Utility Model Publication No. 55-31549, it is also known that an image forming operation is started at a temperature lower than a fixing permitting temperature so that the temperature of the heat roller is increased up to the fixing permitting temperature when a recording sheet actually reaches the heat roller fixing device, thereby, reducing the waiting time more or less. However, the time required to bring the recording sheet to the fixing device after the image forming system starts to feed the recording sheet depends upon a length of the sheet feeding path and the sheet feeding speed of the image forming system, and is about 5-10 seconds at the most. Further, since the heat roller is rotating together with a pressure roller when the recording sheet is brought into the fixing device after the sheet feeding operation is started, the building-up temperature of the surface of the heat roller becomes gentle.
Furthermore, since the building-up temperature of the surface of the heat roller is influenced by the dispersion in the input voltage to the heater and/or in the heating value of the heater itself, in order to ensure that the temperature of the heat roller is increased up to the fixing permitting temperature within the time period when the recording sheet reaches the fixing device after the initiation of the sheet feeding operation, the surface temperature of the heat roller at the end of the waiting time must be set to accommodate the condition that the heating value of the heater is lowest, i.e., a condition that the building-up speed of the surface temperature of the heat roller is slowest. Thus, the waiting time cannot be effectively reduced only by ending the waiting time at a temperature lower than the fixing temperature by an amount of the temperature increase until the recording sheet reaches the fixing device.